Polyolefins, such as polyethylene (PE), are synthesized by polymerizing monomers, such as ethylene (CH2═CH2). Because polyolefins are cheap, safe and stable to most environments and can be easily processed, polyolefin polymers are useful in many applications. For example PE can be classified into several types according to its properties, such as (but not limited to) LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
Polyolefins can be produced from monomers in the presence of diluent and catalyst and optionally one or more co-monomers and molecular weight regulators in a loop reactor. For example, polyethylene can be produced from ethylene monomer in the presence of diluent and catalyst and optionally one or more co-monomers and molecular weight regulators in a loop reactor. Usually the loop reactor is a liquid phase loop reactor wherein the components are circulated under pressure in slurry conditions. The product usually consists of solid particles and is in suspension in a diluent. The liquid diluent can be any hydrocarbon that is inert and liquid during ethylene polymerization, for example alkanes, such as isobutane. The catalyst for producing polyethylene may typically comprise a chromium-based catalyst, a Ziegler-Natta catalyst or a metallocene catalyst. The molecular weight regulator, if added, is usually hydrogen. A co-monomer can be any alpha-olefin with at least three carbons.
Continuous circulation of the slurry contents of the reactor is typically maintained with a pump, which also ensures efficient suspension of the polymer solid particles in the liquid diluent. Circulation is carried out at elevated polymerization temperatures around the loop reactor, thereby producing polyethylene. The product can be discharged by means of settling legs for example, which operate on a batch principle or continuously to recover the product. Settling in the legs is used to increase the concentration of solids in the slurry to be recovered as product slurry. The product can further be discharged to a flash tank, through flash lines, where most of the diluent and unreacted monomers are flashed off and recycled. The polymer particles are dried, optionally additives can be added and finally the polymer is extruded and pelletized. Alternatively, after discharging the product slurry from the settling legs, the reaction mixture may be fed to a second loop reactor serially connected to the first loop reactor where a second polyethylene fraction may be produced. Typically, when two reactors in series are employed in this manner, i.e. a first polyethylene fraction is produced in a first reactor and a second polyethylene fraction is produced in a second reactor, the resultant polyethylene product has a broad or bimodal molecular weight distribution.
Ethylene co-polymerization is the process wherein ethylene is polymerized with co-monomer, i.e. an alpha-olefin, such as e.g. propylene, butene, 1-hexene, etc. The lower the desired density of the final polyethylene, the higher the concentration of co-monomer in the reactor must be. A major problem in such co-polymerization processes is that the control of reaction parameters is very difficult. In particular, the ratio of co-monomer to ethylene monomer can differ at different points in the reactor. It also becomes more difficult to control and optimize reaction conditions, such as reaction temperature and solid concentration in reaction slurry, when producing polyethylene, for example when producing linear low-density polyethylene.
The operating temperature in the reactor is preferably set as high as possible in order to have optimum conditions, i.e. the higher the temperature in the reactor, the higher the productivity of the catalyst. However, increasing the temperature also increases the risk of swelling which may occur in the reactor. Swelling is the phenomenon where diluent enters the amorphous phase of the polymer (that can be partially dissolved in the diluent) and makes the polymer matrix increase in volume (swell). Co-monomer, e.g. 1-hexene, if present, is an even better solvent for low molecular weight polymers, and the polymer produced having a lower density is more prone to swelling (increased ratio of amorphous to crystalline polymer ratio). Hence co-polymerization processes suffer from a higher risk of swelling than homo-polymerization processes. Since some polymer is dissolved in the diluent, the polymer slurry becomes more viscous. Moreover, since swelling leads to an increased volume fraction of solids in the reactor (for the same weight of solids content), it might lead to particles touching each other, therefore increasing the slurry viscosity dramatically, which can perturb the reactor flow, leading to hydrodynamic instabilities and may even lead to blockage of the reactor. Therefore, at least temperature and solid concentration of the slurry must be properly controlled.
In the past, the risk of swelling was decreased by setting the polymerization temperature well below the temperature at which swelling is believed to pose a problem. Classically, this temperature has been predicted for chromium-catalyzed polymerizations by calculating it from the linear relationship between the reaction temperature and the resin density, i.e. the swelling curve. The problem with using traditional swelling curves is that they do not allow the full potential of the catalyst to be exploited. Actual operating temperatures are usually far below the optimum temperatures, which could theoretically still be utilized without the risk of swelling. As a result of the low reactor temperatures, the catalyst has limited productivity, the polymer has difficulties settling in the settling legs and co-monomer is not incorporated efficiently. Furthermore, operating at lower temperatures may impose limitations on the process, as a result of limitations imposed by a cooling system.
EP 1 563 903 describes a means to react to a hydrodynamic instability, it does not provide the means to determine the safe operating region.
EP 0 432 555 A2 establishes control signals which typify flow rate of diluent fluid required to (a) maintain a minimum velocity for the circulating reaction slurry, (b) maintain a maximum pressure head at a selected point in the reactor and (c) maintain a maximum power level supplied to the circulation pump.
US 2015/0209751 discloses a method including measuring parameters for the polymerization reactor including a reactor temperature and a concentration of an induced condensing agent in a polymerization reactor. Induced condensing agents are used in gas phase polymerization to increase cooling capabilities at a given reactor temperature. The use of condensing agents modifies the temperature at which the polymer softens.